Refresh contributor documentation for the current client status

[woksin]

Contributor documentation now matches where Chronicle.Python stands today: no client API yet, no PyPI package, and a development setup you can reproduce from a source checkout.

Added

• A Current status table in the README covering the client API, PyPI availability, the generated contracts package, supported Python versions (3.10 to 3.14), and shared Chronicle documentation, each linked to the issue that tracks it.
• A Check the kernel before writing client code section in the client development guide. It has a curl token check and a contracts-based Python probe that calls EnsureEventStore without and with a bearer token. Both were run against cratis/chronicle:16.38.2-development and print the output the guide shows.
• Troubleshooting for the failures contributors are most likely to hit: blocked downloads of the contracts wheel, a wheel hash mismatch, port 35000 already in use, a kernel that is still starting, an untrusted self-signed certificate, and UNAUTHENTICATED calls.
• The success signal for each required check in CONTRIBUTING.md, plus the Windows PowerShell equivalent of rm -rf dist.

Changed

• The development setup now starts from git clone, notes that the install downloads the contracts wheel from GitHub release assets, and includes an import check that prints the package version.
• The local kernel command pins cratis/chronicle:16.38.2-development, the version the contracts wheel is generated from, and binds only to 127.0.0.1. Compatibility between the 16.38.2 contracts and newer kernels, including latest-development, is unverified, and the guide now says so.
• The TLS guidance names the development certificate's host names and states that the guide does not adopt the .NET client's accept-any-certificate default. How local-development relaxation is exposed remains an API decision for connection-string parsing (Parse and validate Chronicle connection strings #2); relaxed validation must never apply to non-local connections.
• Getting started lists what works today and points readers who need a working client now to the .NET, TypeScript, Kotlin/Java, or Elixir clients.

Fixed

• The client development guide no longer says the contracts dependency will move to PyPI once the trusted-publishing issues are complete. Those issues (Chronicle admin: configure trusted publishing for generated Python contracts #15, Chronicle maintainer: publish the first generated Python contracts release #16) were closed as not planned, so the pinned GitHub release wheel remains the install source with no move to PyPI scheduled.